Username cannot contain repeating underscore or period

Question

I have always struggled with these darn things. I recall a lecturer telling us all once that if you have a problem which requires you use regular expressions to solve it, you in fact now have 2 problems.

Well, I certainly agree with this. Regex is something we don't use very often but when we do its like reading some alien language (well for me anyway)... I think I will resolve to getting the book and reading further.

The challenge I have is this, I need to validate a username based on the following criteria:

1. can contain letters, upper and lower
2. can contain numbers
3. can contain periods (.) and underscores (_)
4. periods and underscores cannot be consecutive i.e. __ .. are not allowed but ._._ would be valid.
5. a maximum of 20 characters in total

So far I have the following : ^[a-zA-Z_.]{0,20}$ but of course it allows repeat underscores and periods.

Now, I am probably doing this all wrong starting out with the set of valid characters and max length. I have been trying (unsuccessfully) to create some look-around or look-behind or whatever to search for invalid repetitions of period (.) and underscore (_) not sure what the approach or methodology to break down this requirement into a regex solution is.

Can anyone assist with a recommendation / alternative approach or point me in the right direction?

Answer 1

This one is the one you need:

^(?:[a-zA-Z0-9]|([._])(?!\1)){5,20}$

Edit live on Debuggex

You can have a demo of what it matches here.

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"Either an alphanum char ([a-zA-Z0-9]), or (|) a dot or an underscore ([._]), but that isn't followed by itself ((?!\1)), and that from 5 to 20 times ({5,20})."

1. (?:X) simply is a non-capturing group, i.e. you can't refer to it afterwards using \1, $1 or ?1 syntaxes.

2. (?!X) is called a negative lookahead, i.e. literally "which is not followed by X".

3. \1 refers to the first capturing group. Since the first group (?:...){5,20} has been set as non-capturing (see #1), the first capturing group is ([._]).

4. {X,Y} means from X to Y times, you may change it as you need.